Design of a Mode-Compressed Wideband Asymmetrical Dipole Antenna With Stable Omnidirectional Radiation Pattern

In this communication, an odd and even mode compression method is proposed to extend the bandwidth of conventional thin dipole antennas while maintaining a stable omnidirectional radiation pattern. Wide bandwidth is achieved by simultaneously exciting and compressing 1^st-order (odd) and 2^nd-order (even) modes, enabling their combination. The offset-fed method is used to simultaneously excite the 1^st- and 2^nd-order modes of the dipole. The 2^nd-order mode is compressed by widening one of the dipole arms, which shifts the 2^nd-order mode towards lower frequencies. The 1^st-order mode can be compressed by shortening the length of the thin arm of the dipole that is not widened, which shifts the 1^st-order mode towards higher frequencies. In addition, to convert the 2^nd-order mode splitting radiation pattern into an omnidirectional one, parasitic strips are loaded on both sides of the thin arm of the dipole. This loading also helps to further compress the 2^nd-order mode towards lower frequencies. The final antenna design features an asymmetric structure. To validate the concept, a prototype was constructed and tested, demonstrating reasonable agreement between the simulated and measured results. The measured impedance bandwidth is 55.6% (1.62-2.87 GHz) and a stable omnidirectional radiation pattern can be maintained across the entire operating frequency band.